Several pharmaceutical manufacturers have recently indicated an interest in obtaining a license for their vWF concentrate or an indication for their Antihemophilic Factor (Human) concentrate as treatment for von Willebrand disease. The development of such products raises issues regarding standardization of assays to be used for assigning potency to these products. A multicenter study has been organized in which 6 proposed vWF products are being assayed in blinded fashion for vWF:Ag, ristocetin cofactor activity (RCof), and vWF multimeric composition using the current WHO plasma standard for FVIII/vWF (87/718). An assessment of within and between laboratory variability for the various assays for vWF:Ag (Laurell, ELISA, IRMA) and RCof (macroscopic, aggregometer) will be made following receipt of data from participating labs. Between laboratory comparisons of vWF multimer determinations will also be made. Data will be analyzed to determine if there is any correlation between vWF:Ag/RCof ratios and multimeric distribution for the various products.